For the past sixty years the petroleum and mining industries have used energy sources to generate surface and sub-surface vibrations within the earth. The reflected energy returning from sub-surface geologic strata is recorded, processed, and analyzed to estimate the most favorable locations to find petroleum or mineral deposits. These efforts are referred to as geophysical or seismic surveys.
Historically, explosives have been the main energy source used for geophysical surveys. These explosives are detonated under accurately timed conditions so that one can measure the elapsed time required for the energy to propagate through the earth and be reflected back to sensitive detectors deployed on or near the earth's surface. These detectors respond to the reflected energy and the vibrations from the reflected energy cause small alternating current voltages that are amplified and then recorded. Accurate estimates can be made about the shape and depth of subterraneum strata and the probability of locating petroleum or mineral deposits from the analysis of this recorded data.
Because the explosive energy is impulsive and non-directional, the signal-to-noise ratios of the recorded data are often too poor to estimate accurately the depth and shape of the desired strata. Thus, other types of energy sources are often used to improve the signal-to-noise ratios in those areas where explosive energy is not effective.
One such energy source is produced by the combination of servo controlled, hydraulically or pneumatically driven vibrator units. It has become the preferred geophysical energy source in many areas of the world. It uses large, self-propelled vehicles that provide a large mass to insure that the vibrator units remain coupled to the earth and produce downward energy.
These vibrator units are driven by electronically produced, oscillatory signals that are amplified by the hydraulic or pneumatic pressure of the unit and cause a baseplate to shake in unison with the applied signals. Normally, these signals are mathematically coded so that the coded information can be removed from the reflected or refracted energy either before, during, or after the data is recorded.
Examples of these vibrator units are disclosed in U.S. Pat. Nos. 3,310,128; 3,718,206; 4,008,784; 4,078,632; and 4,712,641.
U.S. Pat. No. 3,310,128, entitled "Seismic Exploration Methods and Systems," teaches a vehicle that has a tank for holding liquid, a means for supplying and removing liquid to the tank, a repeater gas releasing device, and a means for mounting the device in the tank below the level of liquid.
U.S. Pat. No. 3,718,206, entitled "Amphibious Seismic Exploration Vehicle and Method," teaches a vehicle that provides a seismic signal. That vehicle has a hollow metal casing and pneumatic tires. When the casing and tires are filled with liquid, an acoustic signal transducer transmits a pressure pulse to the surface of the tires. This vehicle can be towed.
U.S. Pat. No. 4,008,784, entitled "Seismic Signal Source," teaches tracked vehicle that provides a seismic signal. That vehicle has a signal means and a conduit means for transferring the seismic signal to a non-compressible fluid within a fluid-filled tire.
U.S. Pat. No. 4,078,632, entitled "Land Seismic Gas Detector," teaches seismic gas detonator. The bottom plate of the detonator has an inwardly-directed slope for increasing the coupling efficiency between the bottom plate and the earth. The detonator is attached to a seismic truck. Part of the detonator is a container which can be made from a car tire, which is maintained upright by lateral support that is pivotally supported by a rod that is hydraulically operated by an inclined cylinder pivotally secured to the truck.
U.S. Pat. No. 4,712,641, entitled "Method and System for Generating Shear Waves and Compression Waves in the Earth for Seismic Surveying," teaches a vehicle that has a liquid-filled cylinder means, an earth-contacting plate mounted in the cylinder, a piston-like reaction mass, and an air gun.
U.S. Pat. Nos. 3,310,128; 3,718,206; 4,008,784; 4,078,632; and 4,712,641 are hereby incorporated by reference for all purposes.
Since these energy sources must be held to the earth, and since the amount of force imparted into the earth is a direct relation to the weight of the mass used, these types of sources cannot be used efficiently in rugged, hostile or impassable terrain, such as a jungle. Thus, large areas of land cannot use these techniques.